feat(integration-excel-agent): populate wrap_plan ExecResult.Files and extract buildFullPlans

[catwithtudou]

What type of PR is this?

• feat

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• This PR title match the format: (optional scope):
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. https://github.com/cloudwego/cloudwego.github.io

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• 功能(integration-excel-agent): 填充 wrap_plan 的执行产物 Files，并抽取 buildFullPlans

(Optional) More detailed description for this PR(en: English/zh: Chinese).

• en:
  • Populate ExecResult.Files in wrap_plan using generic.ListDir(workdir) so the plan output includes deliverable artifacts.
  • Extract buildFullPlans to centralize plan assembly for better reuse and maintainability.
  • Verified locally with temporary unit tests and manual checks of plan.md output; unit test files are not retained in the repository per current repo conventions.
  • No breaking changes; plan.md checklist output format remains unchanged.
  • Note: local validation was run on a recent Go toolchain; please use Go 1.24.7+ if you re-run validation.
• zh(optional):
  • 在 wrap_plan 中通过 generic.ListDir(workdir) 填充 ExecResult.Files，使计划输出包含可交付的执行产物。
  • 抽取 buildFullPlans 以统一计划组装逻辑，提升复用与可维护性。
  • 已在本地通过临时单元测试与手工验证对 plan.md 输出完成校验；根据仓库惯例未保留测试文件。
  • 无破坏性改动；plan.md 清单输出格式保持不变。
  • 如需重新验证，请使用 Go 1.24.7+。

(Optional) Which issue(s) this PR fixes:

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(optional) The PR that updates user documentation:

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